Replace deprecated include qsql.h with qtsqlglobal.h for Qt 6

Fixes MeVisLab#195

Small documentation updates

- changed default Python version from 2.6 to 3.10

Replace INCLUDE by PYTHONQT_INCLUDE env var

- avoids accidental inclusion of system-defined include dirs

Skip alignas specifier

Bring back more Qt5 compatibility

- fix iterator usage as mentioned in review

Add back undefining _POSIX_C_SOURCE and _XOPEN_SOURCE (MeVisLab#106)

- only undefining _POSIX_THREADS had be a problem
- see MeVisLab#103

Limit string representation of argument list in length

otherwise when passing image data to the QImage constructor this can get rather long

CI for CONFIG+=debug + UbSan for tests

Fix UBSAN error for invalid access through pointer. This happens only for Python2 because Py_DECREF is slightly different. Still, I have no opinion if UBSAN is right or wrong here, but this problem occures with the latest XCode/clang available too.